A typical extruder screw comprises a root and a single flight helically extending about the root. The flight forms with the interior cylindrical surface of the barrel a helical channel along which the thermoplastic polymer is conveyed. The screw typically has a feed section, a tapered transition section and a metering section. The feed section is for conveying bulks of raw unmelted polymer from the hopper into the barrel. The tapered transition section is to compensate for the volume decrease as the bulk polymer is compressed and to force the polymer tight against the barrel inner wall for good heat conduction and efficient melting by mechanical shearing. Typically, the polymer begins to melt in the transition section. The necessity for good compaction is especially important in this transition section of the screw to ensure uniform melting of the polymer. The metering section is for advancing the melted polymer to the discharge end of the extruder.
The raw unmelted thermoplastic polymer, typically in the form of powders, pellets, or flakes, is introduced into the feed section of the screw and is advanced along the screw by means of the rotation of the helical flight on the screw. As the solids enter the transition section from the feed section, the solids begin to melt due to the heat created by frictional heat from the rotation of the screw and conductive heat from the heating element on the barrel.
During the course of movement of the solids through the extruder, the solids which are in contact with the hot barrel begin to melt, either by frictional heat or by heat conduction or by both, forming a melt film which adheres to the inner surface of the barrel, and the solids which are not in contact with the hot barrel remain as a solid mass. When the thickness of the melt film exceeds the clearance between the barrel and flight, the leading edge of the flight scrapes the melt film off the inner surface of the barrel and collects it at the forward or leading edge of the flight, forming a pool of the melted polymer. As the solids continue to melt, the width of the pool of melted polymer increases, while the width of the solid mass, normally referred to as the solid bed, decreases, resulting in the breakup of the solid bed into clusters of floating solids in a stream of melted polymer. When this happens, only a small portion, or none of the solid mass is exposed to the barrel and heat must travel from the barrel through the pool of melted polymer to reach the solid mass. As a result, the melting efficiency is greatly diminished once the solids bed is broken up due to the very low thermal conductivity of thermoplastic polymer.
In the extrusion of PVDC resins, the temperature of the polymer melt film near the barrel surface increases rapidly in the early part of the transition section as the compacted solid is forced into the reduced volume of the transition section. This melt film can reach temperatures that will cause localized degradation and subsequent bubbles and dark parabolic lines in a wavy pattern in the film. The temperature buildup is directly related to the peripheral velocity of the polymer at the barrel surface and the pressure generated at this point. The degradation generally occurs in the early part of the transition section where the large compacted solids surface area is rapidly being compressed, thus generating high pressure.
The current solution to this problem is to build low compression screws (compression ratios of less than 3.1) and install dams (restrictions) at the entrance to the metering section or end of the transition section. As used herein, the term "compression ratio" refers to the ratio of feed section flight depth to metering section flight depth. The purpose of such dams, which could extend between a plurality of screw flights, is to restrain and hold back insufficiently or nonuniformly melted polymer particles. An example of such a screw with a dam is shown in the figures of U.S. Pat. No. 3,115,674 to Schrenk et al. These designs have been successful in extruding virgin PVDC resins at high output rates (about 300 pounds per hour) with 3.5 inch diameter screws, but unsuccessful in extruding mixtures of virgin PVDC in powder feed form and a minor amount of PVDC film scraps (recycle).
The film scraps, which are typically compacted into thin flat disks (approximate dimensions of 0.25".times.0.35".times.0.075") float within a stream of spherical powder particles (approximately 220 micrometers in diameter). These film scrap particles, typically only 5% to 10% by weight of the total resin, have a narrower melting peak than the surrounding powder (because they have been melted once already) and also have the tendency to congregate in the center of the solid bed in the screw. This narrower melting point and the recycles being in the center of the solid bed where they cannot receive viscous heat from the barrel/polymer interface, lead to delayed melting of the recycled polymers and subsequent unmelted polymer (gels) being present in the final film article.
It would be desirable to provide an extruder screw which has a high output of extrudate but which also provides close control of the residence time of polymer particles containing minor amounts of recycled resins as they are transported through the screw, resulting in an extrudate which is uniformly melted and which is substantially free of degradation products and/or entrapped air.